1. Field of the Invention
This invention relates to a clogging-resistant entry structure for introducing a particulate solids-containing and/or solids-forming gas stream to a gas processing system.
2. Description of the Related Art
In the processing of particulate solids-containing and/or solids-forming gas streams for treatment thereof, clogging of inlet structures of process equipment with particulate solids from such streams is frequently a problem. As the particulate solids-containing and/or solids-forming gas stream is flowed through the process equipment, solids may be deposited on the surfaces and in the passages of the inlet structure.
In general, particulates associated with the gas stream can come from various sources, including: (i) particulate generated in an upstream process unit which comes downstream to the inlet structure with the gas stream; (ii) particulate formed in the system lines by the reaction of a process gas component with oxygen from leaks coming into the lines; (iii) particulate formed in the system lines due to reaction of two or more process off-gases during flow of the gas stream coming downstream to the inlet structure; (iv) particulate formed by (partial) condensation of off-gases coming downstream to the inlet structure; and (v) particulate formed by reaction of process gases with back-diffusing oxygen or water vapor from a downstream gas stream treatment unit such as for example a downstream water scrubber. In some instances, where the particulate is formed by condensation, it may be possible to ameliorate the problem by heating of process lines, to eliminate the condensable portion of the gas stream. Even with such line heating, however, the problems of particulate from other sources still remain.
Particularly in the field of semiconductor manufacture, inlet clogging is prone to occur from: (a) back-migration to the inlet of water vapor, or liquid water via capillary action, as a combustion product of downstream oxidation operations and/or water scrubbing operations employed to treat the gas stream, causing hydrolysis reactions in a heterogeneous or homogeneous fashion with incoming water-sensitive gases such as Bcl.sub.3, WF.sub.6, DCS, TCS, SiF.sub.4 ; (b) thermal degradation of incoming thermally-sensitive gases; and (c) condensation of incoming gases due to transition points in the system.
The aforementioned inlet clogging problems may require the incorporation of plunger mechanisms or other solids removal means to keep the inlet free of solids accumulations, however these mechanical fixes add considerable expense and labor to the system and may damage the entry over time. In other instances, the inlet clogging problems may be systemic and require periodic preventative maintenance to keep the inlet free of solids accumulations. Such maintenance, however, requires shut-down of the system and loss of productivity in the manufacturing or process facility with which the inlet is associated.
Considering specifically the occurrence of water vapor, or liquid water via capillary action, backstreaming from a downstream water scrubber to an upstream inlet structure in a semiconductor process effluent gas stream treatment system, wherein water vapor released from the scrubber migrates back from the scrubber inlet toward the process tool, against the normal direction of process gas flow, various mechanisms may be involved in the transport of the back-migration of the water vapor.
One mechanism is gas-gas interdiffusion. The only practical way of avoiding this source of water vapor back migration is to add a diffusion boundary to the water scrubber inlet.
Another mechanism for such back-diffusion of water vapor is the so-called Richardson effect annular effect. All dry pumps create a certain amount of pressure oscillation in the gas flow stream. These pressure oscillations create a counterflow transport mechanism that pumps gases against the direction of normal gas flow. This phenomenon is a consequence of the boundary layer annular effect. Because of this effect, the backflow migration velocity is greatest a small distance away from the wall surface.
If particulate solids continue to accumulate with continued operation of process equipment, the inlet structure of such equipment may become occluded to sufficient extent as to plug entirely, or alternatively the solids build-up may not occlude the inlet of the process unit, but may so impair the flows and increase the pressure drop in the system as to render the process equipment grossly inefficient for its intended purpose.
Particularly in the case of water scrubber equipment used for scrubbing of gas streams such as waste gas streams generated in the manufacture of semiconductor devices, the waste gas constituting the influent gas stream to the water scrubber may contain or produce (by reaction or condensation) significant fine particles content, e.g., submicron particles of silica, metals from CVD or other deposition operations, etc. Such waste gas streams tend to clog the inlet of the waste gas water scrubber very readily. As a result the inlet of the water scrubber requires manual cleaning on a frequent basis.
The inlet clogging susceptibility is a major shortcoming of current commercial water scrubber units used in the semiconductor industry. The time required to clog the entry of the water scrubber in such applications is process dependent and site-specific. Among the factors that affect the mean time to failure of the water scrubber due to the clogging of the inlet include: the process tool generating the particulates-containing process effluent stream being treated in the scrubber, the specific process recipes and chemistries employed in the upstream process generating the effluent being treated in the water scrubber, and the character of the inert gas purges used to purge pumps and process lines in the system. Other process conditions and factors are suspected of contributing to or affecting particle build-up in the process system, but are not yet clearly defined. See Abreu, R., Troup, A. and Sahm, M., "Causes of anomalous solid formation in the exhaust systems of low-pressure chemical vapor deposition and plasma enhanced chemical vapor deposition semiconductor processes, J. Vac. Sci. Technol. B 12 (4), July/August 1994, 2763-2767.
It would therefore be a significant advance in the field and is accordingly an object of the present invention to provide an improved inlet structure for a process unit, such as a water scrubber unit, which is resistant to clogging.
It is another object of the present invention to provide such an inlet structure which in addition to being highly resistant to clogging by particulate solids is readily removable from the process equipment with which it is employed, for cleaning purposes.
It is a still further object of the present invention to provide such an inlet structure which is significantly self-cleaning in character.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.